This invention relates to temperature self-regulating electric heating elements and to methods of manufacture of such elements. More particularly, the invention relates to a novel temperature self-regulating electric heating elements employing positive temperature coefficient (PTC) compositions which do not deform during annealing and which are smaller, more flexible, and less expensive to manufacture than similar prior art elements.
PTC compositions and devices which exploit them are well known in the art. The compositions are ideally suited to take part in temperature self-regulating circuit elements, since their electrical conductivity varies with temperature. At relatively low temperatures, a constant voltage applied across a PTC composition evokes a stable flow of current. As the temperature of the PTC composition increases, due to resistive heating, the resistivity gradually increases, until the composition reaches a "switching" temperature or temperature range. At the switching temperature, the compositions' resistance increases dramatically--six fold or more--resulting in greatly decreased current flow.
PTC compositions consist of blends of crystalline polymers and conductive filler materials, such as carbon black. See, e.g., U.S. Pat. No. 3,410,984. Early PTC compositions, such as those disclosed in U.S. Patent No. 3,976,600 required up to 50% conductive carbon black in order to obtain the PTC effect. When such high carbon black loadings were used, a fairly sharp switching temperature was achieved, but the resulting blend was inflexible and deteriorated upon repeated thermal cycling. Lower levels of carbon black loading were tried, as, for example, in U.S. Pat. No. 3,861,029, which disclosed the use of less than 13% by weight carbon black. A drawback with this approach was that the volume resistivity of the carbon loaded material was too high.
A later discovery showed that the volume resistivities of PTC compositions with low levels of carbon black were reduced from approximately 10.sup.9 ohm-cm to 10.sup.3 ohm-cm through annealment at temperatures of 250.degree. F. or higher for periods of about 24 hours. A heating unit constructed utilizing this discovery is disclosed in U.S. Pat. No. 3,861,029 and consists of a polymer-carbon black blend extruded onto a spaced-apart pair of elongate electrodes to form an element dumbbell-shaped in cross section. The extruded PTC formulation both encapsulated and inter-connected the two wires. The two wires were spaced apart by approximately 0.25 inches. The element thereafter was jacketed with a high-melting thermoplastic composition so that when the product reel was annealed, it would not melt, flow, and stick to itself.
The annealment discovery created yet another problem. Specifically, during the annealing phase, when the PTC composition is in a fluid state, the electrodes have greater freedom of motion and are often displaced from their desired positions. An attempt to solve this problem is disclosed in U.S. Pat. No. 4,271,350, where a spacer is inserted between the electrode wires during extrusion of the PTC composition. While a spacer can prevent the two electrode wires from actually touching, the wires may still move very close to each other, causing a hot spot during operation of the heating element. Also, the wires can move away from each other, causing a cool spot during operation of the element.
Prior to the present invention, the only known solutions to the problem of annealment phase electrode displacement--wide electrode separation or the use of spacers --have precluded fabrication of narrow, flexible, and uniformly heated PTC devices. Accordingly, the object of this invention is to provide such a device.